This invention relates to antenna systems, and more particularly to a system for producing a beam with a spherical aperture phase front and a far-field beam with low sidelobes and high efficiency.
Beam efficiency has become an important criteria for microwave antenna systems, particularly for those requiring low noise reception such as radiometry and space telemetry. The objective is to deliver a maximum of radiated energy, considered on a transmit basis, in a predescribed cone. By definition, the cone angle is only 2.5 times the half power beam width.
An obvious design choice is a horn antenna feeding a reflector offset 45.degree.. The reflector can be rotated about the horn axis for beam scanning. Large horn-fed parabolic reflectors are known to have beam efficiencies of approximately 90%. Large Cassegrain systems have beam efficiencies in the order of 85%. High efficiencies are not achieved with these reflector designs because of feed horn sidelobes, spillover past the reflector or reflectors, aperture blockage, diffraction from reflector edges and from aperture blockage, and far field sidelobes of the diffraction pattern of the linear phase aperture field. To achieve an efficiency of 95%, a design must be developed that minimizes or eliminates these effects.
A high beam efficiency of approximately 93% has been achieved utilizing a lens corrected corrugated horn as described by A. F. Kay in U.S. Pat. No. 3,274,603. Briefly, the corrugations suppress illumination in the E plane of the edges of the horn sufficiently for the horn aperture to be illuminated in the E plane with relatively low illumination of the edges similar to that of the H plane. The radiation patterns in both planes are thus made to be similar. This significantly reduced sidelobes and spillover past the parabolic reflector, but the antenna system is still efficiency limited by far-field diffraction sidelobes. The first sidelobe is typically about -20dB and subtracts several percent from beam efficiency.